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What the process capability study has to do with six sigma

Six Sigma and the Process Capability Study both rely on the improvement of customer satisfaction. While Six Sigma is known more for beginning the process of customer satisfaction from the get-go, the Process Capability Study takes a good look at all the processes and then determines how to improve customer satisfaction. Both processes have different ways of going about achieving these results and they are explained below.

The Process Capability has two parts:

  1. Measure the variable of a process
  2. Compare that variable with a proposed specification or product tolerance

The Process Capability study is typically performed for two reasons:

  1. To predict future capability of the process to produce product within specification.

  2. To identify process improvement opportunities.

The process is expected to meet the customer requirements, specifications, or product tolerances. The process relies on a statistical analysis of variation of the process output compared to the allowable specification links. There are 2 forms of Process studies, the Process Potential Study (Ppk) and the Process Capability Study (Cpk).

The Ppk is used to define what has already happened in a single lot of product. Ppk is used in the early phases of a project to obtain an estimate of process variation. It provides a guideline to follow.

The Cpk process is a sampling of a continuous process over planned intervals. It is basically a prediction of what is likely to occur in the future.

Typically the studies results are shown in control charts or as a single number expressed using a process capability index. Both the control charts and the index charts require running the process enough to obtain measurable output to show the process is stable and variability can be reliably estimated.

Six Sigma is primarily focused on eliminating defects to improve all process to a level of 3.4 defects per one million opportunities.

Six Sigma asserts the following:


  • Continuous efforts to reduce variation in process outputs is key to business success

  • - Manufacturing and business processes can be measured, analyzed, improved and controlled

  • - Succeeding at achieving sustained quality improvement requires commitment from the entire organization, particularly from top-level management

Process Capability Study and Six Sigma
Both the Process Capability Study and Six Sigma measure the process and look for ways to improve the process. As a result of this study, some have argued that Six Sigma is a basic approach to improving quality. An analysis done by Charles Holland stated that Six Sigma is "narrowly designed to fix an existing process" in doing so it does not help in "coming up with new products or disruptive technologies." Many do use Six Sigma as a cost-cutting process and not for innovation of new products.

The Process Capability study shows that if process changes are implemented at the right time for the right reasons will have a positive outcome on the product. If done during the study, a process change only introduces error and uncertainty. After the study is completed, you can then introduce changes to improve the process. The end result of the study is designed to meet customer expectations, specifications, and product tolerances.

This is unlike Six Sigma which is a data-driven, systematic approach to problem solving. Six Sigma has the customer in mind at all times and continually looks for ways to improve the overall satisfaction of the customer from the beginning.

Both the Process Capability Study and Six Sigma believe in planning for success. Both processes believe in successful implementation comes from what has been learned in past efforts. While the Process Capability Study looks at what was learned from the study, Six Sigma looks for ways to improve the process from the start.

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